The idea that consciousness has an integrative function has a long
history. The late Francisco Varela and colleagues called it the
"brainweb" (2002). Global Workspace theory suggests a fleeting
memory capacity that enables access between brain functions that are
otherwise separate.

This
makes sense in a brain that is a brainweb, viewed as a massive
parallel distributed system of highly specialized processors. In
such a system coordination and control may take place by way of a
central information exchange, allowing some specialized processors
-- such as sensory systems in the brain -- to distribute
information to the system as a whole. This solution works in
large-scale computer architectures, which show typical "limited
capacity" behavior when information flows by way of a global
workspace. A sizable body of evidence suggests that consciousness is
the primary agent of such a global access function in humans and
other mammals (Baars, 1983, 1988, 1997, 1998).

The "conscious access hypothesis" therefore implies that
consciousness provide a gateway to many capacities of the brain.
Table 1 shows the basic claims made by GW theory.

Conscious perception, inner speech, and visual imagery enable
working memory functions; there is no evidence for unconscious
access to working memory.

3.

Conscious events enable almost all kinds of learning:
episodic and explicit learning, but also implicit and skill
learning.

4.

Conscious perceptual feedback enables voluntary control over
motor functions, and perhaps over any neuronal population and
even single neurons.

5.

Conscious contents can evoke selective attention.

6.

Consciousness enables access
to "self"
-- executive interpreters, located in part in the frontal
cortex.

A number of scientists and philosophers now support some version of
global access. Table 2 shows a dozen of such comments from different
sources, suggesting that the same basic idea emerged independently
from different research traditions. The convergence of ideas is
striking.

Edelman, 1989. Global mapping in a reentrant selectionist model of
consciousness in the brain. (Edelman, 1989)

Dennett, 2001. "Theorists are converging from quite different
quarters on a version of the global neuronal workspace model of
consciousness … On the eve of the Decade of the Brain, Baars
(1988) had already described a 'gathering consensus' in much the
same terms: consciousness, he said, is accomplished by a
"distributed society of specialists that is equipped with a
working memory, called a global workspace, whose
contents can be broadcast to the system as a whole" (p. 42).

Kanwisher, 2001. "… in agreement with Baars (1988), it seems
reasonable to hypothesize that awareness of a particular
element of perceptual information
must entail not just a strong enough neural
representation of information, but also access to that
information by most of the rest of the mind/brain."

Dehaene & Naccache, 2001. "We propose a theoretical framework …
the hypothesis of a global neuronal workspace. …We postulate
that this global availability of information through the
workspace is what we subjectively experience as the conscious
state."

Edelman & Tononi, 2000 p. 148-149: "When we become aware of something … it
is as if, suddenly, many different parts of our brain were
privy to information that was previously confined to some
specialized subsystem. … the wide distribution of information is
guaranteed mechanistically by
thalamocortical and
corticocortical reentry, which facilitates the interactions among distant regions of the brain. "

Llinas et al, 1998. "…
the thalamus represents a hub from
which any site in
the cortex can communicate with any other such site or
sites. … temporal
coincidence of specific and non-specific thalamic activity
generates the functional states that characterize human
cognition.

John, 2000. "Evidence has been steadily accumulating that
information about a stimulus complex is distributed to many
neuronal populations dispersed throughout the brain."

Damasio, 1989. "Meaning is reached by time-locked multiregional
retroactivation of widespread fragment records. Only the latter records can
become contents of consciousness."

While global workspace theory is not the only one to postulate
widespread access associated with consciousness, it develops the
idea in great detail, exploring its implications for learning,
working memory, voluntary control, and self.

A theater metaphor and brain hypotheses:

Like other cognitive architectures, GW theory may be thought of as a
theater of mental functioning. The theater metaphor is too simple,
but it offers a useful first approximation. Consciousness in the
metaphor resembles a bright spot on the stage of immediate memory,
directed there by a spotlight of attention, under executive
guidance. The rest of the theater is dark and unconscious.

This approach leads to specific neural hypotheses. For sensory
consciousness the bright spot on stage is likely to require the
corresponding sensory projection areas of the cortex. Sensory
consciousness in different modalities may be mutually inhibitory,
within approximately 100-ms time cycles. Sensory cortex can be
activated internally as well as externally, resulting in the
"internal senses" of conscious inner speech and imagery. Once a
conscious sensory content is established, it is distributed widely
to a decentralized "audience" of expert networks sitting in the
darkened theater, presumably using corticocortical and
corticothalamic fibers. This is the primary functional role of
consciousness: to allow a theater architecture to operate in the
brain, in order to integrate, provide access, and coordinate the
functioning of very large numbers of specialized networks that
otherwise operate autonomously (Mountcastle, 1978). All the elements
of GW theory have reasonable brain interpretations, allowing us to
generate a set of specific, testable brain hypotheses about
consciousness and its many roles in the brain. Some of these ideas
have now received considerable empirical support (Baars, 2002).

Inner speech, imagery, and Working Memory. Both auditory and visual consciousness
can be activated from the inside of the brain. Inner speech is a
particularly important source of conscious events, and visual
imagery is especially useful for spatial problem solving. The areas
of the left hemisphere involved in outer speech are now known to be
involved in inner speech as well (Paulesu, Frith & Frackowiak,
1993). Likewise, mental imagery is known to involve visual cortex.

Likewise, internally generated feelings may reflect emotional and
motivational processes, including pain, pleasure, hope, fear,
sadness, etc. Such internal sensations may communicate to other
parts of the brain via global broadcasting. Prefrontal executive
systems may not have direct access to action control. Rather, they
may work by evoking motivational imagery, broadcast from the visual
cortex, to control relevant parts of motor cortex, thereby
generating appropriate actions. Parts of the brain that play a role
in emotion may also be triggered by global broadcasting of conscious
contents from sensory cortices and insular cortex. For example, it
is established that the midbrain amygdalae are needed to recognize
facial expressions of fear and anger from the visual system (Le
Doux, 1996). Ultimately such areas, working together, shape actions
controlled by frontal cortex and subcortical automatic systems like
the basal ganglia.

Thus many cortical areas work together to transform goals and
emotions into actions (Baars, 1988). Since there are many spatial
maps throughout the brain, the trade language of the brain may
consist of activated maps, paced by temporal oscillations in the
alpha to gamma spectrum. Such oscillations may coordinate the
activity of multiple sensory, body space, and external spatial maps.

The attentional spotlight. The sensory "bright spot" of
consciousness involves a selective attention system (the theater spotlight), under dual
control of frontal executive cortex and automatic interrupt control from areas such as
the brain stem, pain systems, and emotional centers like the amygdala. It is these
attentional interrupt systems that allow significant stimuli to "break
through" into consciousness in a selective listening task, when the name is spoken
in the unconscious channel.

Context vs. content of sensory experience. A conscious sensory "bright
spot' requires the interaction of sensory analyzers and contextual systems. In
vision sensory content seems to be produced by the ventral visual pathway, while
contextual systems in the dorsal pathway define a spatial domain within which the
sensory event is defined. As pointed out above, parietal cortex is known to include
allocentric and egocentric spatial maps, which are not themselves objects of
consciousness, but which are required to shape every conscious visual event.

There is a major difference between the symptoms of dysfunctional content systems
such as the visual ventral stream, compared to disordered context systems. In the case
of lesioned content, the subject can generally notice a missing part of normal
experience; but for damage to context, the system of experiential expectations is itself
damaged, so that one no longer knows what to expect, and hence what is missing. This may
be why parietal neglect is so often accompanied by anosognosia, a massive but very
specific loss of knowledge about one's body space. Patients suffering from
parietal anosognosia may reject their own body limbs, or see themselves with three
hands. Such specific loss of contextual body information is not accompanied by a loss
of general intelligence or knowledge about the world. Damage to such contextual
knowledge systems may be quite specific.

Self-systems. Cortical activation by a seen object may not be enough to
generate subjective consciousness. The activated visual information may need to be sent
to self-systems, which serve to maintain constancy of an inner framework across
different perceptual situations. When we walk from room to room in a building, we must
maintain a complex and multi-leveled organization that can be viewed in Global Workspace
theory as a higher-level context. Major goals, for example, are not changed when we walk
from room to room, but conscious perceptual experiences do change. Gazzaniga (1996) has
found a number of conditions under which split-brain patients encounter conflict between
right and left hemisphere executive and perceptual functions. He has proposed the
existence of a "narrative self" in the left frontal cortex, based on
split-brain patients who are clearly using speech in the left hemisphere to talk to
themselves, sometimes try to force the right hemisphere to obey its commands. When that
proves impossible, the left hemisphere will often rationalize or reinterpret the
sequence of events so as to repair its understanding of the interhemispheric
conflict.

Analogous repairs of reality are observed in other forms of brain damage, such as
parietal neglect. They commonly occur whenever humans are confronted with major,
unexpected life changes. The left-hemisphere narrative interpreter may be considered as
a higher-level context system that maintains expectations and intentions across many
specific situations. While the narrative stream itself is conscious, it is shaped by
unconscious contextual executive influences.

If we consider Gazzaniga's narrative interpreter of the left hemisphere to be
one kind of self-system in the brain, it must receive its own flow of sensory input.
Visual input from one half of the field may be integrated in one visual hemisphere, as
described above, under retinotopic control from area V1. But once it comes together in
visual cortex, it needs to be conveyed to frontal areas on the left side of the brain,
in order to inform the narrative interpreter of the current state of perceptual affairs.
The left prefrontal self system then applies a host of criteria to the input, such as
"Did I intend this result?" "Is it consistent with my current and
long-term goals?" "If not, can I reinterpret the input to make sense
in my running account of reality?"

It is possible that the right hemisphere has a parallel system that does not speak,
internally or externally, but that may be more able to deal with anomalies via irony,
jokes, and other emotionally sophisticated strategies. The evidence appears to be good
that the right prefrontal cortex can understand such figurative uses of language, while
the left does not.

Full consciousness may not exist without the participation
of such prefrontal self systems.

New brain evidence for global distribution of conscious contents. On the basis
of psychological evidence, GW theory predicted that conscious contents are widely
distributed in the brain (Baars, 1988, 1997). Today that case is supported by a sizable
body of brain evidence, at least for sensory consciousness (Baars, 2002). For example,
Dehaene and colleagues showed that backward-masked visual words evoked brain activity
confined to the well-known visual word recognition areas of cortex (Dehaene et al,
2001). Identical conscious words triggered higher levels of activity in these areas, but
more importantly, they evoked far more widely distributed activity in parietal and
prefrontal cortex. That result has now been replicated a dozen times, using different
brain imaging techniques, different experimental comparisons between conscious and
unconscious input (e.g. binocular rivalry, inattentional blindness, neglect,
extinction), and different modalities (audition, pain, vision, and sensorimotor tasks).
In all cases conscious sensory input evoked far wider and more intense brain activity
than identical unconscious input.

These findings support the general claim that conscious stimuli mobilize large areas
of cortex, presumably to distribute information about the stimuli. This idea is
essential to the following discussion. If consciousness serves to mobilize many
unconscious specialized networks, the active elements of mental life that always need to
be conscious -- input, recall, rehearsal, inner speech, visual imagery and report --
may be widely distributed in order to recruit specific unconscious functions needed to
carry out those tasks.

Evolutionary considerations

A number of neurobiologists suggest that sensory consciousness emerged with early
mammals or transitional reptiles (therapsids), who evolved a large thalamocortical
complex (e.g. Edelman & Tononi, 2000). Working memory -- what may be called
"extended consciousness" beyond the immediate contents -- involves two
endogenous kinds of sensory consciousness, the phonological loop (for inner speech), and
the visual sketchpad (for voluntary visual imagery). The phonological loop seems to use
the classic speech regions of the left hemisphere, Broca's and Wernicke's
areas. Wernicke's area is located near auditory cortex, and in the case of inner
speech it seems to support internalized auditory speech perception. Likewise, visual
imagery appears to involve visual cortex, especially when it is vividly conscious.

Working memory involves the purposeful use of such endogenous sensory consciousness.
It is not clear when the purposeful use of visual imagery and inner speech evolved, but
it plausibly did so after the major growth of prefrontal cortex and speech areas
associated with hominid evolution -- that is, in the last few million years. Prefrontal
cortex is key to voluntary goals in humans, and speech is of course the basis of the
phonological loop involved in mental rehearsal.

In contrast, spontaneous imagery may have appeared much earlier -- such as the
visual image of a lion evoked by the sound of a lion's roar. Such
spontaneous imagery may be common among mammalian prey animals, while predators may have
spontaneous visual images of prey that can be smelled but not seen. Certainly the dream
state, characterized by rich conscious visual imagery, evolved with early mammals.

Thus both external and endogenous sensory capacities may have evolved millions of
years before the hominid capacities needed for human mental life. These considerations
suggest that endogenous sensory consciousness long precedes specifically human mental
capacities, and that the distinctive hominid development of human thought depends on
voluntary control of these pre-existing functions. This is especially true of language
and its role in voluntary control of inner speech and purposeful visual imagery.

Summary and Conclusions

Global Workspace theory suggests that consciousness enables multiple networks to
cooperate and compete in solving problems, such as retrieval of specific items from
immediate memory. The overall function of consciousness is to provide widespread
access, which in turn may serve functions of coordination and control. Consciousness is
the gateway to the brain, enabling control even of single neurons and whole neuronal
populations (Baars, 1988). None of these control functions become directly conscious, of
course, but conscious feedback seems required to recruit control by prefrontal networks.
In the metaphor of the theater, it is as if each specialized audience member can decide
locally whether or not to be driven by input from the bright spot on stage. Executive
functions -- the director behind the scenes -- are also largely unconscious, often
using the actor in the spotlight on the stage of working memory capacity to recruit and
trigger specific functions.

Footnotes

1. Global workspace theory emerged from the cognitive architecture tradition
pioneered by Alan Newell and Herbert A. Simon. Newell and his coworkers were the first
to show the utility of a global workspace capacity in a complex system of specialized
knowledge sources, which could cooperatively solve problems no single constituent could
solve alone. The empirical connection with consciousness was not made in this tradition,
however.